1. Field of the Invention
The present invention relates to apparatus for treating semiconductor wafers utilizing a plasma generated by a microwave discharge in a magnetic field such as an electron cyclotron resonance discharge.
2. Description of the Prior Art
In the production of semiconductor devices such as transistors and integrated circuits, semiconductor substrates or wafers are subjected to treatments such as thin film formation, etching, oxidation and doping. Recently, wafer treating apparatus utilizing a plasma generated by an electron cyclotron resonance (ECR) discharge, which is a kind of microwave discharge in a magnetic field, have been developed. This apparatus has a number of advantages over conventional chemical vapor deposition apparatus, including low operating temperatures and high treatment quality.
U.S. Pat. No. 315730 teaches a fundamental structure of such semiconductor wafer treating apparatus utilizing a plasma generated by an electron cyclotron resonance discharge. As shown in FIG. 4 of the drawings, such an apparatus generally comprises a wafer treating chamber 1 accommodating a susceptor 8 for a wafer 9, and a cylindrical plasma generating chamber 2 disposed on top of the wafer treating chamber 1. Microwave energy generated by a microwave source (not shown) is introduced into the plasma generating chamber 2 through a rectangular waveguide 3 having a rectangular cross section, a rectangular-to-circular microwave converter, and a quartz plate 2e. Further, a solenoidal electromagnetic coil 5 is provided which surrounds the plasma generating chamber 2 for forming a magnetic field in the plasma generating chamber 2 and the wafer treating chamber 1. A gas introducing port 6 and a gas exhausting port 7 are formed in the ceiling of the plasma generating chamber 2 and in the floor of the wafer treating chamber 1, respectively.
The operation of the apparatus shown in FIG. 4 is as follows. After the gas remaining in the chambers 1 and 2 is thoroughly exhausted through the port 7, a reactive gas is introduced into the chambers 1 and 2 through the port 6, a part of the gas being exhausted through the port 7 to keep the pressure thereof in the chambers 1 and 2 at a predetermined level. Next, microwaves having a frequencey of 2.45 GHz generated by the microwave source is supplied to the plasma generating chamber 2 through the waveguide 3 and the converter 4. At the same time, the coil 5 is energized to produce a magnetic field in the plasma generating chamber 2 and the wafer treating chamber 1; the flux density of the magnetic field in the plasma generating chamber 2 is regulated to 875 G to cause electron cyclotron resonance therein in cooperation with the microwave energy; the magnetic field produced by the coil 5 diverges from the plasma generating chamber 2 toward the susceptor 8.
Thus, the electrons in the plasma generating chamber 2 are accelerated in helical paths in electron cyclotron resonance absorbing the microwave energy in the chamber 2; the collisions of these fast moving electrons generate a dense gas plasma in the plasma generating chamber 2. The plasma thus generated is conveyed to the wafer 9 along the diverging lines of the magnetic field produced by the solenoidal coil 5. Thus, a treatment of the wafer, e.g. thin film formation or etching, is effected. As is well known, the kind of the gas utilized in the treatment or the pressure thereof, the power of the microwave source, etc., are chosen according to the type of treatment which is effected on the wafer.
The conventional ECR plasma wafer treating apparatus as described above, however, has the following disadvantages.
Microwave energy in a circulat TE.sub.11 mode is directly supplied to the plasma generating chamber 2 in the conventional apparatus. The lines of force E of the electric field of the microwave supplied to the plasma generating chamber 2, however, are distributed as shown in FIG. 5. That is, the density of the lines is high near the axis of the chamber 2 (i.e., the electric field strength is great thereat) and low near the circumference thereof (i.e., the electric field strength is small thereat). It also varies along the circular paths running in the circumferential direction of a plane perpendicular to the axis of the chamber 2. Thus, the plasma density becomes spatially uneven in the plasma generating chamber 2 so that the quantity of reactive ions arriving at the semiconductor wafer 9 varies from one location to another on the surface of the wafer 9 which is detrimental to the uniformity of the wafer treatment.